Although flame retardant polymer compositions have been in use for several decades, they have generally relied upon the presence of halogens, mainly chlorine or bromine containing chemicals, to yield flame retardancy. Examples include polychloroprene ("neoprene"), chlorosulphonated polyethylene ("Hypalon"), thermoplastic PVC compounds, or compounds of polyethylene with halogenated flame retardant additives.
Materials under development over the last ten years, and even more intensively in the last five years in Europe and the United States have focused on halogen free, flame retardant (HFFR) compounds, since halogens give off very toxic and corrosive combustion products in fires.
Thermoset HFFR's have been developed based on polyethylene and its copolymers, but these materials have inherently high processing costs due to the need for crosslinking. Thermoplastic HFFR's, usually classified as thermoplastic polyolefins or TPO's since they are typically based on polymers and copolymers of ethylene and propylene, are the focus of much industrial materials research at present for construction and transportation because of their greater ease of use (fabrication into end use products) and the recyclability of trim or scrap.
Development of a cost effective and acceptable performance HFFR/TPO is a challenging project. The essence of the technical challenge is as follows: Conventional materials, namely PVC compounds, have shown a good balance of properties in mechanical strength and flexibility, chemical, aging resistance and low cost. Unfortunately, burning PVC's release a great deal of black smoke and their combustion fumes contain HCl gas, which is highly corrosive, particularly in combination with the water used to fight fires. This hydrochloric acid is capable of destroying expensive computer equipment and even such rugged electrical fixtures as fuse boxes which may not be directly destroyed by the flame or heat of a fire.
Halogen free systems of modest cost are restricted to polyolefins in terms of raw material. Polyolefins do not have inherently good flame resistance. Choice of halogen free flame retardant additives is limited to certain hydrated halogen free flame retardant additive minerals such as alumina trihydrate (ATH) or magnesium hydroxide, which are relatively inexpensive.
These flame retardant additives function by releasing their water of hydration, preferably at temperatures above those required for processing but below those of combustion of the flame retardant composition. At relatively high concentrations, such additives also impair combustion by conducting heat relatively efficiently from burning surfaces. To maximize these flame-retardant effects, it is preferable that the flame retardant additives be present at maximum levels.
However, these particular halogen free flame retardant materials are relatively inefficient and must be added in large amounts (&gt;50% by weight). Because such flame retardant materials are non reinforcing in the final product, HFFR/TPO compounds using this conventional technology normally have poor strength and flexibility, poor processing characteristics (eg. ease of mixing and extrusion) and only fair flame retardancy.
Approaches tried to avoid these difficulties include the use of coupling agents (to compensate for the non-reinforcing nature of ATH), and intumescent additives (also called char formers).
Silicone flame retardants for plastic compositions have been extensively investigated in U.S. Pat. No. 4,387,176. A composition which lends fire-retardancy to a thermoplastics includes a silicone with a group IIA metal organic salt and a silicone resin which is soluble in the silicone to impart flame-retardancy to the thermoplastic. The silicone is of the general organopolysiloxane group, such as polydimethylsiloxane. The silicone resin is generally represented by the formula MQ where M is the monofunctional group of the average formula R.sub.3 SiO.sub.0.5 and the tetrafunctional Q units of the average formula SiO.sub.2 with the average ratio of approximately 0.3:4 of M units per Q unit. The patent teaches that this fire-retardant composition is useful with a variety of thermoplastics including specifically polypropylene, polyethylene, polycarbonate, polystyrene, acrylonitryl-butadiene-styrene terpolymer, polyphenylene oxide-polystyrene blends, acrylic polymer, polyurethane and polyamides. It is required that the group IIA metal organic salts be included to ensure solubility of the silicone resin in the polysiloxane base. Representative salts include magnesium stearate, calcium stearate, barium stearate, strontium stearate. It has been found, however, that compositions of this type are not readily processable particularly when high loadings of fire-retardant additives are included.